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Controversy regarding the formation of the Mount Isa copper orebodies in the ‘silica dolomite’ has been a little more difficult to settle, but in spite of some dissenters It is now considered that conditions necessary for deposition of all sulphides began with the development, through penecontemporaneous faulting, of a restricted basin of sedimentation.
At Mount Isa subsequent post-depositional deformation and metamorphism has recrystallised, folded and partly re-distributed the metal sulphides, but bedding features such as particle size grading are still evident.Structural studies have revealed that following the cessation of sedimentation, the Mount Isa Group sediments were subjected to two episodes of deformation which tilted, folded and faulted the rock sequence.Accompanying metamorphism was extremely mild in the immediate area of Mount Isa and eastwards, but more severe to the west of a major fault zone which had developed just west of the Mount Isa deposit (the Mount Isa Fault).Most geologists today agree that the orebodies at Mount Isa were originally deposited at the same time as, and as part of, the host shales, being deposited by submarine hot, salty, metal and sulphide-rich volcanic “springs”. 1) in Middle Proterozoic sediments of the Precambrian shield region of north-west Queensland, Mount Isa is one of the world’s largest concordant base metal deposits.Since the host shales are also highly fossiliferous, it will be argued that they were laid down rapidly in the year-long global catastrophe of Noah’s Flood in situations similar to the biblically described “fountains of the deep”. Silver-lead-zinc and copper orebodies found in the same beds but spatially independent of each other, extend over a strike length of 4.5 km, a width of 1 km and a depth of 1.6 km with an average dip of 65°W (See Fig. Since mining began in 1931 close to 80 million tonnes [as at February 1984] of ore averaging 3% copper, and another 55 million tonnes averaging 178 g/tonne silver, 7.4% lead and 5.8% zinc have been produced, but proven reserves are currently in excess of 140 million tonnes at 3% copper and 60 million tonnes of silver-lead-zinc ore.Early investigators concluded that the silver-lead-zinc orebodies were epigenetic, that is, the mineralisation had been introduced by hydrothermal fluids (that is, hot waters) into the structurally prepared host rocks subsequent to the latter’s deposition.
Galena, sphalerite and pyrite were said to have been deposited by hydrothermal replacement of selected beds within the shales, despite the theoretical necessity for complex alternation of deformation and introduction of fluids to explain mineral textures.
They found these algal stromatolites both immediately below the ore-bearing Urquhart Shale in the Native Bee Siltstone, and above in the Kennedy-Spear Siltstone.
They described them and concluded that their shape and internal fabric closely resemble algal structures that occur today in shallow marine environments such as at Hamelin Pool, Shark Bay, Western Australia, and the Persian Gulf, while similar stromatolites are also known on the shores of the Great Salt Lake, Utah, and in the Coorong area, South Australia.
They were particularly interested in trying to assess if pyrite in the shales had a microbial origin, and described a number of forms, some of which represented the organic framework from which they had dissolved the pyrite.
Other forms appeared to be double-walled cells, commonly infilled with pyrite, which Love and Zimmerman, however, has demonstrated conclusively that the host Urquhart Shale contains abundant organic remains of micro-organisms.
The fossiliferous character of the ore-bearing Urquhart Shale and upper Mount Isa Group sediments has thus been established beyond doubt. old rocks at Mount Isa have identical counterparts alive today, it is more than reasonable to conclude that these fossils were deposited recently and contemporaneously with both the Urquhart Shale and the Mount Isa ore deposit during Noah’s Flood.